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Cobalt complexes are well-known catalysts for photocatalytic proton reduction in water. Macrocyclic tetrapyridyl ligands (pyrphyrins) and their CoII complexes emerged in this context as a highly efficient class of H2 evolution catalysts. On the basis of this framework, a new macrocyclic CoII complex consisting of two keto-bridged bipyridyl units (Co diketo-pyrphyrin) is presented. The complex is synthesized along a convenient route, is well soluble in water, and shows high activity as a water reduction catalyst (WRC). In an aqueous system containing [Ru(bpy)3]Cl2 as a photosensitizer and NaAscO as a sacrificial electron donor, turnover numbers (TONs) of 2500 H2/Co were achieved. Catalysis is terminated by a limited electron supply and decomposition of the photosensitizer but not of the WRC, highlighting the distinct stability of Co diketo-pyrphyrin.

Cobalt complexes are well-known catalysts for photocatalytic proton reduction in water. Macrocyclic tetrapyridyl ligands (pyrphyrins) and their Co

The synthesis of the two penta-pyridyl type ligands pyridine-2,6-diylbis(dipyridin-2-ylmethanol) (PPy, 1) and bis-2,2''-bipyridine-6-yl(pyridine-2-yl)methanol (aPPy, 2) is described. Both ligands coordinate rapidly to the 3d element cations Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II), thereby yielding complexes of the general composition [MBr(1)](+) and [MBr(2)](+), respectively. Further, the X-ray structures of selected complexes with ligands 1 and 2 are described. They show metal center dependent structural features and complexes with 2 exhibiting distinctly distorted octahedral geometries. Moreover, photocatalytic water reduction with [Co(II)Br(PPy)]Br (1c) and [Co(II)Br(aPPy)]Br (2c) as water reducing catalysts (WRC) was investigated. Both complexes showed catalytic activity in water when in presence of ascorbic acid as sacrificial electron donor and [Re(py)(bpy)(CO)3](+) (3) as photosensitizer (PS). Turnover numbers, TONs (H2/Co), up to 11,000 were achieved. Complex 2c was more active than 1c, whereas none of the other complexes showed any activity.

The mechanism of photocatalytic hydrogen production was studied with a three-component system consisting of fac-[Re(py)(CO)3bipy](+) (py = pyridine, bipy = 2,2'-bipyridine) as photosensitizer, [Co(TPY-OH)(OH2)](2+) (TPY-OH = 2-bis(2-pyridyl)(hydroxy)methyl-6-pyridylpyridine), a polypyridyl-based cobalt complex, as water reduction catalyst (WRC), and triethanolamine (TEOA) as sacrificial electron donor in aqueous solution. A detailed mechanistic picture is provided, which covers all processes from excited state quenching on the time scale of a few nanoseconds to hydrogen release taking place between seconds and minutes at moderately basic reaction conditions. Altogether these processes span 9 orders of magnitude in time. The following reaction sequence was found to be the dominant pathway for hydrogen generation: After reductive quenching by TEOA, the reduced photosensitizer (PS) transfers an electron to the Co(II)-WRC. Protonation of Co(I) yields Co(III)H which is reduced in the presence of excess Co(I). Co(II)H releases hydrogen after a second protonation step, which is detected time-resolved by a clark-type hydrogen electrode. Aside from these productive steps, the role of side and back reactions involving TEOA-derived species is assessed, which is particularly relevant in laser flash photolysis measurements with significantly larger transient concentrations of reactive species as compared to continuous photolysis experiments. Most notable is an equilibrium reaction involving Co(I), which is explained by a nucleophilic addition of Co(I) to the oxidation product of TEOA, an electrophilic iminium ion. Quantum chemical calculations indicate that the reaction is energetically feasible. The calculated spectra of the adduct are consistent with the spectroscopic observations.